doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com psi on hingh pin

Thanks Rocketman!

Re the shear stress on the cross pin. We are all assuming that the firing force is directed equally onto the breech face by the cartridge and onto the pin.

But, the pin is rarely unsupported in a double gun. A fair amount of metal is in FRONT of the pin, the knuckle, in full contact, thus any force exerted on the pin will be channeled to the action body. The same goes for OUs with bifurcated pins, they are supported at the front via protrusions of the action body metal. The pin undergoes deformation, more than shear stress. It is worth pondering whether this deformation comes from the forces of firing or via the recovery phase of the action body. If the action flexes back then it must spring back to its original form, squeezing the barrels between breech face and cross pin.

Personally I disagree with the assumption (fostered by Greener, Burrard and Thomas) that the cartridge head acts alone in pressing against the breech face. Whatever happens during firing is, I believe, far more complex than a simple thrust on the breech face by the cartridge head, and an equally direct and simple thrust on the cross pin. Over the years I have seen some strange damage to double guns but have yet to see a sheared cross pin, even in folding shotguns that have totally unsupported cross pins.

Good points, SL.



You never will, either. We are talking about double shear strength here, not single shear. For the hinge pin to shear it would have to shear at both ends, simultaneously. Ain't gonna happen from the forces generated by firing a shotshell.

I am about as far from an engineer as one man can be, but I deal with double shear situations daily on the farm. We pull subsoilers as deep as 18" in compacted soil. It is being held in place, until it hits an obstruction (stump, rock) by a Gr5 1/2" dia. bolt that does not have any tension on it from a nut, i.e. it is not being "squeezed together" to increase the shear strength. Gr5 is very similar to 4140 in strength, according to the charts I have looked at. Using a simple analogy of a 3/8" cross pin, or hinge pin, it doesn't take an engineer to see that there will never be a sheared doublegun hinge pin caused by any force that can be contained in the chamber and fired off a man's shoulder.

Chuck, I absolutely agree. I was not trying for a correct number. Rather, I was working on a ballpark SWAG. Nothing more.

DDA

SL, I think you have a point on how the barrel acts. I spent some time talking to a group of engineering students (one ME senior with fresh mech. of materials) about the likely behaviour of the chamber area under firing pressure and decay. They agree that it will not react strictly radially. However, we did not come up with any estimate of the magnitude of axial force. So, we all need to do a bit of research on this issue. Good thinkin' on this issue!!

DDA

Given that paper and tape shims work, if only as a temporary fix, I would guess there is not be much stress on the pin.

I think the force on the hook is mainly due to the mass of the barrels accelerating during recoil.

The barrel tubes want to basically stay motionless during firing. The reaction on the breech face pushes action and stock into your shoulder. As the action and stock go back, the barrels must hang on for the ride. This acceleration and puts force on the pin. For the same load, a heavy set of barrels will stress the pin more than a lighter set.

So I guess we could calculate the recoil energy, weight the various components of a gun, and by conservation of energy determine the force on the connections holding all the parts together.

If I am correct, the distance over which recoil is resisted will greatly affect the acceleration force on the pin. A lead sled contraption that sharply halts recoil will put more stress on the pin.

I say this is a lot like the forces put on rifle scope mounts.

Update. I ran a stress analysis for a typical 12 bore chamber with an inside radius of 0.405", outside radius of 0.600", and a radius within the wall of 0.500" at a pressure of 10,000 psi. The hoop stress is 20,400 psi, the axial stress is 8370 psi, and the radial stress (within the wall) is -3680 psi. All the above are well within the working stress levels for low carbon steel.

Gotta think about these numbers. What are they going to tell us?? Knowledge and/or opinions welcome.

DDA

Don,
There's some good S-N stuff on this link.

http://www.mscsoftware.com/training_vide...heory.15.3.html

The first chart on page 43 of this link is nice and straight forward for discussion.

http://books.google.com/books?id=Hbo8dI4...eel&f=false



I like the explanation of fatigue at this link.
http://www.sv.vt.edu/classes/MSE2094_NoteBook/97ClassProj/anal/kelly/fatigue.html#S_Ncurve